1. Field of the Invention
The present invention relates to a process for the production of low-temperature firing ceramic compositions which are used as, for example, dielectric ceramics for microwave resonators, high-frequency filters and monolithic capacitors, and ceramics for multilayered substrates.
2. Description of the Related Art
To miniaturize electronic parts such as microwave resonators and high-frequency filters, efforts have been made to replace cavity resonators with dielectric ceramics each having a high dielectric constant. Such a dielectric resonator utilizes the effect that the wavelength of an electromagnetic wave is shortened inside a dielectric to 1/.epsilon..sup.1/2 the wavelength in free space, wherein .epsilon. is the dielectric constant of the dielectric.
However, a dielectric ceramic having such a temperature coefficient such as to be applicable as a dielectric resonator can only have a specific dielectric constant .epsilon.r of at most 100, which is insufficient to meet the requirements of further miniaturization of resonators in recent years.
An effective solution to this requirement under the limitation of low specific dielectric constant .epsilon.r of dielectric ceramics is to use LC resonators which are employed in microwave circuits. In addition, the application of a lamination forming process, which has practically been applied to monolithic capacitors and multilayer substrates, to the construction of LC resonant circuits can give further miniaturized electronic parts having high reliability.
To obtain LC resonators each having a high Q-value in a microwave band by the lamination forming process, the internal electrodes to be integrated in monolithic capacitors or multilayer substrates must have a high electrical conductivity. In other words, such internal electrodes must be composed of gold, silver, copper or another metallic material having a high electrical conductivity, which internal electrodes are cofired with dielectric ceramics or ceramics for multilayer substrates.
Thus, the dielectric materials need to be high in dielectric constant, Q-value and theimostability and to be low-temperature firing ceramic compositions which can be cofired with internal electrodes composed of a metallic material having a low melting point. Few dielectric materials, however, provide these requirements in good balance.
As a possible solution to this problem, for example, Japanese Unexamined Patent Publication No. 6-40767 discloses a technique including the steps of: (1) calcining a porcelain composition mainly containing a BaO--TiO.sub.2 --ReO.sub.3/2, wherein Re is a rare earth element, at a temperature of 1050.degree. C. or higher, milling the calcined composition to a mean particle diameter of 0.8 .mu.m or less, and adding a glass powder mainly containing B.sub.2 O.sub.3 to give a material powder; or (2) adding a glass powder predominantly comprising B.sub.2 O.sub.3 to a porcelain composition containing a BaO--TiO.sub.2 --ReO.sub.3/2 as a main component, calcining the resultant mixture at a temperature of 1050.degree. C. or higher, and milling the calcined mixture to a mean particle diameter of 0.8 .mu.m or less to give a material powder; and forming and firing the material powder to give a low-temperature firing ceramic composition.
According to this technique, a dielectric porcelain composition which can be sintered at a temperature equal to or lower than the melting point of silver and has a comparatively high specific dielectric constant, a high Q-value and a low temperature coefficient of a resonant frequency is realized. However, the technique, in which a main crystalline phase (e.g., Ba(Nd,Bi).sub.2 Ti.sub.4 O.sub.12) is not precipitated in the calcination at a low temperature of about 1000.degree. C., requires the calcination at a high temperature of 1050.degree. C. or higher (preferably 1100.degree. C. to 1300.degree. C.), and the calcination at such a high temperature entails high costs.